1. Field of the Invention
The present invention relates generally to a sheet delivery device for sheet-fed machines, and more particularly concerns a sheet stacking device for use in printing presses.
2. Description of the Prior Art
Various types of sheet delivery devices are known in the art. Generally, these include a stack-receiving tray or pallet for receiving a stack of sheets. The stack-receiving pallet is lowered until the stack reaches a predetermined height. The stack-receiving pallet is then removed, and an empty one substituted. To avoid having to stop sheet-feeding during this change, a secondary stack-receiving device is generally provided. This secondary stack-receiving device accumulates sheets until the new stack-receiving pallet is in place, whereupon these sheets are deposited on the stack and normal operation continues. An arrangement of this kind is known from DE-PS 1 231721 wherein the secondary stack-receiving device is in the form of a roller shutter which is unwindable from a winding shaft disposed near the back of the delivery mechanism and whose side edges run in stationary horizontal guides. The roller shutter has a relatively large mass and, therefore, can move only comparatively slowly. Consequently, the output rate of the sheet-fed machine has to be reduced upon activation of the secondary stack-receiving device. However, it is inevitable that a number of sheets arrive before termination of the entry operation of the roller shutter so that there are bound to be relative movements between the shutter and the bottom sheet and, therefore, relative movement between the sheets above the bottom sheet, with the risk of damage to the sheets and the freshly printed image. This disadvantage is further heightened in the known arrangement because the roller shutter is required to bridge substantially the whole length of the stack, so that the required entry time is quite long. Another disadvantage of the known arrangement is that when the shutter withdraws, the secondary stack thereon may be skewed because of friction. Also, the shutter is in the form of a bulky coil when wound and may hinder access to the delivery mechanism from the front thereof.